Ecology — populations, cycles and human impact.

HookThe wolves that changed the rivers

ModelLevels of organisation — from one organism to a whole ecosystem

Ecology is built from nested levels, and getting the vocabulary exactly right is worth easy marks. An individual organism belongs to a population — all the members of one species in a habitat. All the populations of different species living and interacting in the same area form a community. Add the non-living surroundings — soil, water, air, light — and you have an ecosystem: the community plus its abiotic environment, interacting as a system.

The idea binding a community together is interdependence. Species depend on each other for food, shelter, pollination and seed dispersal, so removing one can pull down many. A stable community is one where the species and environmental factors are in balance, so population sizes stay roughly constant over time. Feeding relationships are drawn as food chains that always begin with a producer — usually a green plant or alga that photosynthesises to make its own food and, in doing so, captures the energy every other organism will later use. Producers are eaten by primary consumers (herbivores), then secondary and tertiary consumers (predators), with an apex predator at the top. Predator and prey numbers rise and fall in linked cycles offset in time: more prey feeds more predators, whose rising numbers then cut the prey back, which later starves the predators — peaks chasing each other with a lag.

MechanismAbiotic and biotic factors — the two things that move a population

A population's size is pushed around by two kinds of factor. Abiotic factors are the non-living conditions: light intensity, temperature, moisture level, soil pH and mineral content, wind intensity and direction, the concentration of carbon dioxide for plants, and the level of dissolved oxygen for aquatic animals. Change one and you change which organisms thrive — more light means faster photosynthesis and more plant growth, which supports more herbivores.

Biotic factors are the living pressures: availability of food, the arrival of a new predator, a new pathogen to which the population has no resistance, and competition. Competition is the one examiners lean on. Plants compete for light and space, and for water and mineral ions from the soil; animals compete for food, mates and territory. When two species need the same limited resource, the better-adapted one can outcompete the other so badly that the loser's numbers collapse locally — the grey squirrel displacing the red across most of Britain is the standard case, helped by a pathogen (squirrelpox) the greys carry but survive. Notice how often a real answer needs both categories at once: the red squirrel's decline is competition and disease (both biotic), but its last strongholds are set by conifer forest, which is abiotic.

MechanismAdaptations — structural, behavioural and functional

Organisms survive their conditions because of adaptations — features that develop over evolutionary time and come in three flavours. Structural adaptations are physical: a polar bear's thick fat layer and small surface-area-to-volume ratio conserve heat; a cactus stores water in a swollen stem and shrinks its leaves to spines to cut water loss. Behavioural adaptations are things an organism does: desert lizards shelter at midday and hunt at dawn; birds migrate. Functional adaptations happen inside the body: a camel produces very concentrated urine and tolerates a large rise in body temperature before it sweats, saving water.

The organisms that force the definition are extremophiles — species adapted to conditions that would kill almost anything else. Bacteria living around deep-sea hydrothermal vents cope with very high temperatures, crushing pressure and high salt concentrations, using functional adaptations in their enzymes and membranes that keep working where ordinary proteins would denature. In an exam, always label which type of adaptation you are describing and link it to the abiotic factor it answers: 'spines instead of leaves (structural) reduce surface area, cutting water loss in a dry climate' scores where 'it has spines' does not.

DataRequired practical 7 — sampling with quadrats and transects

You cannot count every organism in a field, so ecologists sample. Required practical 7 uses a quadrat — a square frame of known area — placed at random positions to estimate abundance, and a transect — a line across a habitat with quadrats at set intervals — to study how distribution changes along an environmental gradient, such as light under a tree canopy or exposure up a rocky shore. The independent variable is usually distance or an abiotic factor you measure with a sensor; the dependent variable is the number or percentage cover of a species; you control the method by keeping quadrat size and counting rules the same.

Randomisation is the point students miss. Placing quadrats 'where the plants are' is biased sampling and inflates the estimate; instead you generate random coordinates over a gridded area so every spot has an equal chance. The main errors are too few quadrats (large sampling error), non-random placement (systematic bias) and miscounting organisms that straddle the frame edge — you improve reliability by taking more quadrats and calculating a mean. For mobile animals the equivalent is capture–recapture, where population size ≈ (first catch × second catch) ÷ number of marked animals recaptured in the second catch.

ModelCycling of materials — carbon and water going round again

Energy flows through an ecosystem once and is lost as heat, but the atoms are used again and again — B7 focuses on the carbon and water cycles. In the carbon cycle, carbon dioxide is removed from the air by photosynthesis and locked into plant compounds; it is returned by respiration in plants, animals and microorganisms, by combustion when fuels and wood burn, and by decomposition. When organisms die, decomposers — bacteria and fungi — break down their bodies and release carbon dioxide back to the air and mineral ions back to the soil for plants to reuse. Over millions of years, undecomposed matter can become fossil fuels, taking carbon out of circulation until we burn it.

The water cycle provides the fresh water land organisms need. Energy from the Sun evaporates water from oceans and land; plants add water vapour by transpiration; the vapour rises, cools and condenses into clouds, then falls as precipitation, some running back to the sea and some soaking into the ground. The rate of decomposition — and so how fast material is recycled — depends on temperature, water availability and oxygen: warm, moist, well-aerated conditions speed microbes up, which is exactly why a compost heap rots faster in summer than a fridge ever will.

CaseBiodiversity, waste and land use — the human footprint

Biodiversity is the variety of all the different species of organisms on Earth, or within an ecosystem. High biodiversity makes an ecosystem more stable, because species are less dependent on any single other one and the web can absorb shocks. The blunt problem is that human activity is reducing biodiversity, and three pressures do most of the damage.

The first is waste and pollution. A rising human population and a higher standard of living mean more resources are used and more waste is produced; unless it is handled properly, that waste pollutes water (with sewage, fertiliser run-off and toxic chemicals), land (with landfill and toxic chemicals such as pesticides and herbicides) and air (with smoke and acidic gases). The second is land use: humans reduce the land available for other organisms by building, quarrying, farming and dumping waste. Destroying peat bogs to sell peat as compost is a double hit — it removes a rare habitat and releases the carbon dioxide stored in the peat as it decomposes or burns, adding to the very warming discussed next. Every one of these trades a short-term human benefit against a long-term loss of the living systems we depend on.

CaseDeforestation, global warming and maintaining biodiversity

Deforestation — clearing large areas of forest, especially in the tropics — is done for timber, to grow crops for biofuels, and to make grazing land for cattle. It hurts three ways: fewer trees means less carbon dioxide removed by photosynthesis; burning and the decay of felled wood release stored carbon dioxide; and the loss of a hugely diverse habitat drives species toward extinction. Those extra greenhouse gases feed global warming. Levels of carbon dioxide and methane in the atmosphere are rising, and the scientific consensus, based on peer-reviewed evidence, is that this is increasing global temperatures — roughly 1.1 °C above pre-industrial levels, with atmospheric carbon dioxide up from about 280 parts per million before industrialisation to over 420 today. The consequences examiners want are concrete: melting ice and rising sea levels, changes to the distribution of species as regions warm, changes to migration patterns, and a further reduction in biodiversity.

Maintaining biodiversity is the counter-move, and the specification lists the programmes: breeding endangered species in captivity, protecting and regenerating rare habitats, reintroducing field margins and hedgerows around fields where farmers grow only one crop, reducing deforestation and greenhouse-gas emissions, and recycling resources rather than sending them to landfill. The honest tension to flag in an evaluation is cost: these measures compete with farming, housing and profit, which is exactly why they are contested rather than automatic.

VocabularyKey terms the mark scheme pays for

TrapsMisconceptions that cost marks

ExamWhat examiners want